Clemens, in U.S. Pat. Nos. 3,842,194, 3,842,217 and 3,909,517, discloses an information recording and playback system which utilizes variable capacitance. In one configuration of the Clemens system, information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. The disc record is overcoated with a conductive layer followed by a dielectric layer and then a lubricant layer. For example, groove widths of about 2.6 micrometers and groove depths of about 0.5 micrometer may be used. During playback a pickup stylus tip about 2.0 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick, engages the groove as the record is rotated on a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the pre-recorded information.
In a different capacitive information disc record structure, the disc record is grooveless. Signals recorded in the disc record are used to keep the playback stylus properly positioned.
Recent developments have resulted in the use of a conductive disc record substrate which eliminates the need for conductive and dielectric layers. This substrate may be used with either grooved or nongrooved structures.
Kiezer, in U.S. Pat. No. 4,162,510, discloses a playback stylus which may be used with a grooved capacitive video disc record. The stylus is keel-tipped and comprises a dielectric support element having a constricted terminal portion and shoulders interconnecting the body with a constricted terminal portion. The constricted terminal portion is defined by a prow, a substantially flat rear surface remote from said prow, side surfaces extending from the side edges of the rear surface, a bottom extending from the bottom edge of the rear surface and a plurality of additional surfaces extending from the prow and intersecting the bottom and side surfaces. The maximum separation between the substantially parallel side surfaces is the given groove width.
Leedom et al., in a co-pending application entitled, "CONFORMAL VIDEO DISC STYLUS", Ser. No. 158,943, filed June 10, 1980, disclose an improved method for fabricating a playback video disc stylus for use with a non-trapezoidal spiral-grooved information disc record. The method includes preparing a conductive layer on a surface of the dielectric support element, shaping the dielectric support element so that the conductive layer is remote from the prow, and conformally lapping the bottom surface of the stylus with an abrasive silicon oxide layer so that the bottom surface conforms to the groove shape of the information record.
A problem which occurs in the manufacture of a capacitive video disc stylus is determining the width of the bottom surface of the stylus. If the width of the bottom surface is greater than the groove width, the stylus may pick up signals from adjacent spiral groove turns. These extraneous signals result in noticeably poorer video and audio quality. If the width of the bottom surface is too small, the signal-to-noise ratio of the video and audio signals may be significantly reduced.
Measurement of the width of the stylus is difficult since the desired stylus width is generally about 2 micrometers and the desired accuracy of measurement is within a range of better than .+-.0.2 micrometer. The use of a microscope is time consuming and thus, not cost effective for large scale production. It is therefore desirable to have an improved method to determine the width of the bottom surface of these styli.